This invention relates generally to linear feedback servo control systems and more particularly concerns such control systems as applied to the generation of ophthalmic lenses.
Typically, on substantially linear feedback control systems equipped with a linear servo algorithm such as a proportional integral derivative or PID loop, feed forward terms are introduced to minimize following error during continuous motion. These systems frequently employ both velocity feed forward and acceleration feed forward terms to obtain accuracy as close to ideal as possible for the intended bandwidth of motion. The ideal condition for best accuracy would be to have a zero phase response or lag and an amplitude response of unity or 0 dB throughout the bandwidth of intended motion for that axis. Normally in such systems the phase response can be tuned by adjusting the feed forward parameters so that, at lower frequencies, approximately no phase lag is observed. When this is done, the amplitude response often resembles a high pass or sometimes a low pass filter. However, the amplitude response at the very lowest frequencies will be approximately unity or 0 dB.
On systems involving the synchronized motion of multiple axes, the phase response in the desired frequency bandwidth is obtained by adjusting the feed forward parameters of each axis. The result is that each axis has good phase response at lower frequencies but has an amplitude response resembling a filter. Unfortunately, the filter characteristics for the amplitude response of each axis are different. Quite often, it is required that one, or several, of the synchronized axes have motion at higher than the very lowest frequencies where, as stated above, the amplitude response of each axis is already approximately unity. Typically, this is the case for mechanisms involving a fast tool servo. In order to achieve ideal conditions for accuracy throughout the system, the amplitude response of the fast moving axes should be altered to be approximately equal to unity in the intended motion bandwidth for those axes while at the same time maintaining approximately zero phase lag for all synchronized axes in their respective intended motion bandwidths.
It is, therefore, an object of this invention to provide a multiple axis linear feedback servo control system suitable for the generation of an ophthalmic lens. Another object of this invention is to provide a multiple axis linear feedback servo control system which coordinates the phase responses of the synchronized motion axes. A further object of this invention is to provide a multiple axis linear feedback servo control system which applies similar filter characteristics to the servo mechanisms controlling the synchronized motion axes. A further object of this invention is to provide a multiple axis linear feedback servo control system which alters the amplitude response of fast moving axes to improve accuracy over the intended motion bandwidth. And, it is an object of this invention to provide a multiple axis linear feedback servo control system which maintains an approximately zero phase lag for all synchronized motion axes when the amplitude response of the fast moving axes have been altered to improve accuracy over the intended motion bandwidth.